Work in a warm or hot environment coupled with high metabolic loads and/or work in protective clothing can bring about considerable heat strain (e.g. increased heart rate and core body temperature). Acclimation via increasing exposure to work in a hot environment is one potent control used to reduce the level of heat stress and decrease the signs of heat strain. Documented physiological responses of acclimation include improved circulatory efficiency and thermoregulation. Once workers are acclimated to work in the heat, physiological adaptations will persist as long as workers remain in the hot environment. However, workers may be absent from the hot environment due to training, vacation, illness or normal shift change, and some adaptations may be lost requiring some degree of re-acclimation. However, the rate of decay of acclimation is uncertain. In addition, re-acclimation will be dependent on the degree of decay. The basic philosophy of the experimental design is to determine the rate of decay of physiological adaptations of acclimation and the time to re-establish the physiological changes of acclimation. The first goal is to determine loss pattern of acclimation in 7-day increments over a period of approximately 6 weeks. The second goal of the study is to identify the decay pattern from the previous data collected. Based on this information, the loss pattern for 1/3 and 2/3 of total loss of acclimation will be determined. These time periods will be used for the planned decay periods and the time to full re-acclimation will be determined for each. These goals can be accomplished with an experimental design that systematically determines loss of acclimation and time to re-establish acclimation.